Method and apparatus for reporting channel state information for fractional beamforming in a wireless communication system

ABSTRACT

A method and apparatus for reporting Channel State Information (CSI) by a User Equipment (UE), for fractional beamforming using a massive antenna array of a Base Station (BS) in a wireless communication system are disclosed. The method includes receiving information about a plurality of Reference Signal (RS) resources from the BS, generating CSI including a sub-precoder for at least one of the plurality of RS resources, and a Channel Quality Indicator (CQI) and a Rank Indicator (RI) for all of the plurality of RS resources, and reporting the CSI to the BS. The massive antenna array is divided by rows or columns into partitions and the plurality of RS resources correspond to the partitions.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for reporting Channel StateInformation (CSI) for fractional beamforming in a wireless communicationsystem.

BACKGROUND ART

A brief description will be given of a 3rd Generation PartnershipProject Long Term Evolution (3GPP LTE) system as an example of awireless communication system to which the present invention can beapplied.

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an exemplary wirelesscommunication system. The E-UMTS system is an evolution of the legacyUMTS system and the 3GPP is working on the basics of E-UMTSstandardization. E-UMTS is also called an LTE system. For details of thetechnical specifications of UMTS and E-UMTS, refer to Release 7 andRelease 8 of “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network”, respectively.

Referring to FIG. 1, the E-UMTS system includes a User Equipment (UE),an evolved Node B (eNode B or eNB), and an Access Gateway (AG) which islocated at an end of an Evolved UMTS Terrestrial Radio Access Network(E-UTRAN) and connected to an external network. The eNB may transmitmultiple data streams simultaneously, for broadcast service, multicastservice, and/or unicast service.

A single eNB manages one or more cells. A cell is set to operate in oneof the bandwidths of 1.25, 2.5, 5, 10, 15 and 20 Mhz and providesDownlink (DL) or Uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be configured so as to providedifferent bandwidths. An eNB controls data transmission and reception toand from a plurality of UEs. Regarding DL data, the eNB notifies aparticular UE of a time-frequency area in which the DL data is supposedto be transmitted, a coding scheme, a data size, Hybrid Automatic RepeatreQuest (HARD) information, etc. by transmitting DL schedulinginformation to the UE. Regarding UL data, the eNB notifies a particularUE of a time-frequency area in which the UE can transmit data, a codingscheme, a data size, HARQ information, etc. by transmitting ULscheduling information to the UE. An interface for transmitting usertraffic or control traffic may be defined between eNBs. A Core Network(CN) may include an AG and a network node for user registration of UEs.The AG manages the mobility of UEs on a Tracking Area (TA) basis. A TAincludes a plurality of cells.

While the development stage of wireless communication technology hasreached LTE based on Wideband Code Division Multiple Access (WCDMA), thedemands and expectation of users and service providers are increasing.Considering that other radio access technologies are under development,a new technological evolution is required to achieve futurecompetitiveness. Specifically, cost reduction per bit, increased serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, etc. arerequired.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ona method and apparatus for reporting Channel State Information (CSI) forfractional beamforming in a wireless communication system.

Technical Solution

The object of the present invention can be achieved by providing amethod for reporting Channel State Information (CSI) by a User Equipment(UE), for fractional beamforming using a massive antenna array of a BaseStation (BS) in a wireless communication system, including receivinginformation about a plurality of Reference Signal (RS) resources fromthe BS, generating CSI including a sub-precoder for at least one of theplurality of RS resources, and a Channel Quality Indicator (CQI) and aRank Indicator (RI) for all of the plurality of RS resources, andreporting the CSI to the BS. The massive antenna array is divided byrows or columns into partitions and the plurality of RS resourcescorrespond to the partitions.

The generation of the CSI may include generating the CSI, assuming thatone linking precoder for linking the plurality of RS resources is apredetermined value or a random value. Or the method may further includereceiving information about one linking precoder for linking theplurality of RS resources from the BS.

If the linking precoder is assumed to be a random value, the generationof the CSI may include generating CQIs, assuming that a plurality oflinking precoder candidates are applied respectively, and calculating anaverage or a minimum value of the CQIs as the CQI for all of theplurality of RS resources.

In another aspect of the present invention, provided herein is areception apparatus in a wireless communication system, including awireless communication module configured to receive information about aplurality of RS resources from a transmission apparatus that performsfractional beamforming using a massive antenna array and to transmit CSIgenerated using the plurality of RS resources to the transmissionapparatus, and a processor configured to generate the CSI including asub-precoder for at least one of the plurality of RS resources, and aCQI and an RI for all of the plurality of RS resources. The massiveantenna array of the transmission apparatus is divided by rows orcolumns into partitions and the plurality of RS resources correspond tothe partitions.

The processor may generate the CSI, assuming that one linking precoderfor linking the plurality of RS resources is a predetermined value or arandom value. Or the wireless communication module may receiveinformation about one linking precoder for linking the plurality of RSresources from the transmission apparatus.

If the linking precoder is assumed to be a random value, the processormay generate CQIs, assuming that a plurality of linking precodercandidates are applied respectively, and may calculate an average or aminimum value of the CQIs as the CQI for all of the plurality of RSresources.

If the 2D antenna array is divided by columns into the partitions, thesub-precoder may be used for vertical beamforming and a linking precodermay be used for horizontal beamforming. Or If the 2D antenna array isdivided by rows into the partitions, the sub-precoder may be used forhorizontal beamforming and a linking precoder may be used for verticalbeamforming.

Further, each of the partitions may include the same number of antennaports and a gap between the antenna ports may be equal to or smallerthan a predetermined value.

Advantageous Effects

According to embodiments of the present invention, CSI for fractionalbeamforming can be reported efficiently in a wireless communicationsystem.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an example of a wirelesscommunication system;

FIG. 2 illustrates a control-plane protocol stack and a user-planeprotocol stack in a radio interface protocol architecture conforming toa 3rd Generation Partnership Project (3GPP) radio access networkstandard between a User Equipment (UE) and an Evolved UMTS TerrestrialRadio Access Network (E-UTRAN);

FIG. 3 illustrates physical channels and a general signal transmissionmethod using the physical channels in a 3GPP system;

FIG. 4 illustrates a structure of a radio frame in a Long Term Evolution(LTE) system;

FIG. 5 illustrates a structure of a downlink radio frame in the LTEsystem;

FIG. 6 illustrates a structure of an uplink subframe in the LTE system;

FIG. 7 illustrates a configuration of a general Multiple Input MultipleOutput (MIMO) communication system;

FIGS. 8 and 9 illustrate downlink Reference Signal (RS) configurationsin an LTE system supporting downlink transmission through four antennas(4-Tx downlink transmission);

FIG. 10 illustrates an exemplary downlink Demodulation Reference Signal(DMRS) allocation defined in a current 3GPP standard specification;

FIG. 11 illustrates Channel State Information-Reference Signal (CSI-RS)configuration #0 of downlink CSI-RS configurations defined in a current3GPP standard specification;

FIG. 12 illustrates antenna tilting schemes;

FIG. 13 is a view comparing an antenna system of the related art with anActive Antenna System (AAS);

FIG. 14 illustrates an exemplary AAS-based User Equipment (UE)-specificbeamforming;

FIG. 15 illustrates an AAS-based two-dimensional beam transmissionscenario;

FIG. 16 illustrates an example of applying aligned fractional precodingto a uniform linear array according to another embodiment of the presentinvention;

FIG. 17 illustrates an example of applying columnwise aligned fractionalprecoding to a square array according to another embodiment of thepresent invention;

FIG. 18 illustrates an example of applying rowwise aligned fractionalprecoding to a square array according to another embodiment of thepresent invention;

FIG. 19 illustrates an example of applying row group-wise alignedfractional precoding to a square array according to another embodimentof the present invention;

FIGS. 20, 21, and 22 illustrate methods for allocating a pilot patternaccording to a third embodiment of the present invention; and

FIG. 23 is a block diagram of a communication apparatus according to anembodiment of the present invention.

BEST MODE

The configuration, operation, and other features of the presentinvention will readily be understood with embodiments of the presentinvention described with reference to the attached drawings. Embodimentsof the present invention as set forth herein are examples in which thetechnical features of the present invention are applied to a 3rdGeneration Partnership Project (3GPP) system.

While embodiments of the present invention are described in the contextof Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, they arepurely exemplary. Therefore, the embodiments of the present inventionare applicable to any other communication system as long as the abovedefinitions are valid for the communication system. In addition, whilethe embodiments of the present invention are described in the context ofFrequency Division Duplexing (FDD), they are also readily applicable toHalf-FDD (H-FDD) or Time Division Duplexing (TDD) with somemodifications.

The term ‘Base Station (BS)’ may be used to cover the meanings of termsincluding Remote Radio Head (RRH), evolved Node B (eNB or eNode B),Reception Point (RP), relay, etc.

FIG. 2 illustrates control-plane and user-plane protocol stacks in aradio interface protocol architecture conforming to a 3GPP wirelessaccess network standard between a User Equipment (UE) and an EvolvedUMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is apath in which the UE and the E-UTRAN transmit control messages to managecalls, and the user plane is a path in which data generated from anapplication layer, for example, voice data or Internet packet data istransmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transferservice to its higher layer, a Medium Access Control (MAC) layer. ThePHY layer is connected to the MAC layer via transport channels. Thetransport channels deliver data between the MAC layer and the PHY layer.Data is transmitted on physical channels between the PHY layers of atransmitter and a receiver. The physical channels use time and frequencyas radio resources. Specifically, the physical channels are modulated inOrthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL)and in Single Carrier Frequency Division Multiple Access (SC-FDMA) forUplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, aRadio Link Control (RLC) layer via logical channels. The RLC layer at L2supports reliable data transmission. RLC functionality may beimplemented in a function block of the MAC layer. A Packet DataConvergence Protocol (PDCP) layer at L2 performs header compression toreduce the amount of unnecessary control information and thusefficiently transmit Internet Protocol (IP) packets such as IP version 4(IPv4) or IP version 6 (IPv6) packets via an air interface having anarrow bandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (orL3) is defined only on the control plane. The RRC layer controls logicalchannels, transport channels, and physical channels in relation toconfiguration, reconfiguration, and release of radio bearers. A radiobearer refers to a service provided at L2, for data transmission betweenthe UE and the E-UTRAN. For this purpose, the RRC layers of the UE andthe E-UTRAN exchange RRC messages with each other. If an RRC connectionis established between the UE and the E-UTRAN, the UE is in RRCConnected mode and otherwise, the UE is in RRC Idle mode. A Non-AccessStratum (NAS) layer above the RRC layer performs functions includingsession management and mobility management.

DL transport channels used to deliver data from the E-UTRAN to UEsinclude a Broadcast Channel (BCH) carrying system information, a PagingChannel (PCH) carrying a paging message, and a Shared Channel (SCH)carrying user traffic or a control message. DL multicast traffic orcontrol messages or DL broadcast traffic or control messages may betransmitted on a DL SCH or a separately defined DL Multicast Channel(MCH). UL transport channels used to deliver data from a UE to theE-UTRAN include a Random Access Channel (RACH) carrying an initialcontrol message and a UL SCH carrying user traffic or a control message.Logical channels that are defined above transport channels and mapped tothe transport channels include a Broadcast Control Channel (BCCH), aPaging Control Channel (PCCH), a Common Control Channel (CCCH), aMulticast Control Channel (MCCH), a Multicast Traffic Channel (MTCH),etc.

FIG. 3 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, theUE performs initial cell search (S301). The initial cell search involvesacquisition of synchronization to an eNB. Specifically, the UEsynchronizes its timing to the eNB and acquires a cell Identifier (ID)and other information by receiving a Primary Synchronization Channel(P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a DownLinkReference Signal (DL RS).

After the initial cell search, the UE may acquire detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources forsignal transmission to the eNB, the UE may perform a random accessprocedure with the eNB (S303 to S306). In the random access procedure,the UE may transmit a predetermined sequence as a preamble on a PhysicalRandom Access Channel (PRACH) (S303 and S305) and may receive a responsemessage to the preamble on a PDCCH and a PDSCH associated with the PDCCH(S304 and S306). In the case of a contention-based RACH, the UE mayadditionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S307) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S308), which is a general DL and UL signal transmission procedure.Particularly, the UE receives Downlink Control Information (DCI) on aPDCCH. Herein, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information that the UE transmits to the eNB on the UL orreceives from the eNB on the DL includes a DL/UL ACKnowledgment/NegativeACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPPLTE system, the UE may transmit control information such as a CQI, aPMI, an RI, etc. on a PUSCH and/or a PUCCH.

FIG. 4 illustrates a structure of a radio frame used in the LTE system.

Referring to FIG. 4, a radio frame is 10 ms (327200×T_(s)) long anddivided into 10 equal-sized subframes. Each subframe is 1 ms long andfurther divided into two slots. Each time slot is 0.5 ms (15360×T_(s))long. Herein, T_(s) represents a sampling time and T_(s)=1/(15kHz×2048)=3.2552×10⁻⁸ (about 33 ns). A slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain. In the LTE system, one RB includes 12 subcarriersby 7 (or 6) OFDM symbols. A unit time during which data is transmittedis defined as a Transmission Time Interval (TTI). The TTI may be definedin units of one or more subframes. The above-described radio framestructure is 2.0 purely exemplary and thus the number of subframes in aradio frame, the number of slots in a subframe, or the number of OFDMsymbols in a slot may vary.

FIG. 5 illustrates exemplary control channels included in a controlregion of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first oneto three OFDM symbols of a subframe are used for a control region andthe other 13 to 11 OFDM symbols are used for a data region according toa subframe configuration. In FIG. 5, reference characters R1 to R4denote RSs or pilot signals for antenna 0 to antenna 3. RSs areallocated in a predetermined pattern in a subframe irrespective of thecontrol region and the data region. A control channel is allocated tonon-RS resources in the control region and a traffic channel is alsoallocated to non-RS resources in the data region. Control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carryinginformation about the number of OFDM symbols used for PDCCHs in eachsubframe. The PCFICH is located in the first OFDM symbol of a subframeand configured with priority over the PHICH and the PDCCH. The PCFICHincludes 4 Resource Element Groups (REGs), each REG being distributed tothe control region based on a cell Identity (ID). One REG includes 4Resource Elements (REs). An RE is a minimum physical resource defined byone subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4according to a bandwidth. The PCFICH is modulated in Quadrature PhaseShift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)indicator channel carrying an HARQ ACK/NACK for a UL transmission. Thatis, the PHICH is a channel that delivers DL ACK/NACK information for ULHARQ. The PHICH includes one REG and is scrambled cell-specifically. AnACK/NACK is indicated in one bit and modulated in Binary Phase ShiftKeying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor(SF) of 2 or 4. A plurality of PHICHs mapped to the same resources forma PHICH group. The number of PHICHs multiplexed into a PHICH group isdetermined according to the number of spreading codes. A PHICH (group)is repeated three times to obtain a diversity gain in the frequencydomain and/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDMsymbols of a subframe. Herein, n is 1 or a larger integer indicated bythe PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carriesresource allocation information about transport channels, PCH andDL-SCH, a UL scheduling grant, and HARQ information to each UE or UEgroup. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, aneNB and a UE transmit and receive data usually on the PDSCH, except forspecific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data andinformation indicating how the UEs are supposed to receive and decodethe PDSCH data are delivered on a PDCCH. For example, on the assumptionthat the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked byRadio Network Temporary Identity (RNTI) “A” and information about datatransmitted in radio resources (e.g. at a frequency position) “B” basedon transport format information (e.g. a transport block size, amodulation scheme, coding information, etc.) “C” is transmitted in aspecific subframe, a UE within a cell monitors, that is, blind-decodes aPDCCH using its RNTI information in a search space. If one or more UEshave RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicatedby “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 6, a UL subframe may be divided into a control regionand a data region. A Physical Uplink Control Channel (PUCCH) includingUplink Control Information (UCI) is allocated to the control region anda Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. The middle of the subframe is allocated tothe PUSCH, while both sides of the data region in the frequency domainare allocated to the PUCCH. Control information transmitted on the PUCCHmay include an HARQ ACK/NACK, a CQI representing a downlink channelstate, an RI for Multiple Input Multiple Output (MIMO), a SchedulingRequest (SR) requesting UL resource allocation. A PUCCH for one UEoccupies one RB in each slot of a subframe. That is, the two RBsallocated to the PUCCH are frequency-hopped over the slot boundary ofthe subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocatedto a subframe in FIG. 6.

Now a description will be given of a MIMO system. MIMO can increase thetransmission and reception efficiency of data by using a plurality ofTransmission (Tx) antennas and a plurality of Reception (Rx) antennas.That is, with the use of multiple antennas at a transmitter or areceiver, MIMO can increase capacity and improve performance in awireless communication system. The term “MIMO” is interchangeable with‘multi-antenna’.

The MIMO technology does not depend on a single antenna path to receivea whole message. Rather, it completes the message by combining datafragments received through a plurality of antennas. MIMO can increasedata rate within a cell area of a predetermined size or extend systemcoverage at a given data rate. In addition, MIMO can find its use in awide range including mobile terminals, relays, etc. MIMO can overcome alimited transmission capacity encountered with the conventionalsingle-antenna technology in mobile communication.

FIG. 7 illustrates the configuration of a typical MIMO communicationsystem. Referring to FIG. 7, a transmitter has N_(T) Tx antennas and areceiver has N_(R) Rx antennas. The use of a plurality of antennas atboth the transmitter and the receiver increases a theoretical channeltransmission capacity, compared to the use of a plurality of antennas atonly one of the transmitter and the receiver. The channel transmissioncapacity increases in proportion to the number of antennas. Therefore,transmission rate and frequency efficiency are increased. Given amaximum transmission rate R_(o) that may be achieved with a singleantenna, the transmission rate may be increased, in theory, to theproduct of R_(o) and a transmission rate increase rate R₁ in the case ofmultiple antennas. R₁ is the smaller value between N_(T) and N_(R).

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was verified in themiddle 1990s, many techniques have been actively proposed to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards such asstandards for 3G mobile communications, future-generation Wireless LocalArea Network (WLAN), etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many aspects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system with N_(T) Tx antennas and N_(R) Rxantennas as illustrated in FIG. 7 will be described in detail throughmathematical modeling. Regarding a transmission signal, up to N_(T)pieces of information can be transmitted through the N_(T) Tx antennas,as expressed as the following vector.

s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given as

ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . x_(N) _(T) may be generated bymultiplying the transmission power-controlled information vector ŝ by aweight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These N_(T) transmission signals arerepresented as a vector X, which may be determined by [Equation 5].Herein, w_(ij) denotes a weight between a j^(th) piece of informationand an i^(th) Tx antenna and W is referred to as a weight matrix or aprecoding matrix.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In general, the rank of a channel matrix is the maximum number ofdifferent pieces of information that can be transmitted on a givenchannel, in its physical meaning. Therefore, the rank of a channelmatrix is defined as the smaller between the number of independent rowsand the number of independent columns in the channel matrix. The rank ofthe channel matrix is not larger than the number of rows or columns ofthe channel matrix. The rank of a channel matrix H, rank(H) satisfiesthe following constraint.

rank(H)≦min(N _(T) ,N _(R))  [Equation 6]

A different piece of information transmitted in MIMO is referred to as‘transmission stream’ or shortly ‘stream’. The ‘stream’ may also becalled ‘layer’. It is thus concluded that the number of transmissionstreams is not larger than the rank of channels, i.e. the maximum numberof different pieces of transmittable information. Thus, the channelmatrix H is determined by

# of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 7]

“# of streams” denotes the number of streams. One thing to be notedherein is that one stream may be transmitted through one or moreantennas.

One or more streams may be mapped to a plurality of antennas in manyways. The stream-to-antenna mapping may be described as followsdepending on MIMO schemes. If one stream is transmitted through aplurality of antennas, this may be regarded as spatial diversity. When aplurality of streams are transmitted through a plurality of antennas,this may be spatial multiplexing. Needless to say, a hybrid scheme ofspatial diversity and spatial multiplexing in combination may becontemplated.

It is expected that the future-generation mobile communication standard,LTE-A will support Coordinated Multi-Point (CoMP) transmission in orderto increase data rate, compared to the legacy LTE standard. CoMP refersto transmission of data to a UE through cooperation from two or moreeNBs or cells in order to increase communication performance between aUE located in a shadowing area and an eNB (a cell or sector).

CoMP transmission schemes may be classified into CoMP-Joint Processing(CoMP-JP) called cooperative MIMO characterized by data sharing, andCoMP-Coordinated Scheduling/Beamforming (CoMP-CS/CB).

In DL CoMP-JP, a UE may instantaneously receive data simultaneously fromeNBs that perform CoMP transmission and may combine the receivedsignals, thereby increasing reception performance (Joint Transmission(JT)). In addition, one of the eNBs participating in the CoMPtransmission may transmit data to the UE at a specific time point(Dynamic Point Selection (DPS)).

In contrast, in downlink CoMP-CS/CB, a UE may receive datainstantaneously from one eNB, that is, a serving eNB by beamforming.

In UL CoMP-JP, eNBs may receive a PUSCH signal from a UE at the sametime (Joint Reception (JR)). In contrast, in UL CoMP-CS/CB, only one eNBreceives a PUSCH from a UE. Herein, cooperative cells (or eNBs) may makea decision as to whether to use CoMP-CS/CB.

Now a detailed description will be given of RS.

In general, a transmitter transmits an RS known to both the transmitterand a receiver along with data to the receiver so that the receiver mayperform channel measurement in the RS. The RS indicates a modulationscheme for demodulation as well as the RS is used for channelmeasurement. The RS is classified into Dedicated RS (DRS) for a specificUE (i.e. UE-specific RS) and Common RS (CRS) for all UEs within a cell(i.e. cell-specific RS). The cell-specific RS includes an RS in which aUE measures a CQI/PMI/RI to be reported to an eNB. This RS is referredto as Channel State Information-RS (CSI-RS).

FIGS. 8 and 9 illustrate RS configurations in an LTE system supportingDL transmission through four antennas (4-Tx DL transmission).Specifically, FIG. 8 illustrates an RS configuration in the case of anormal CP and FIG. 9 illustrates an RS configuration in the case of anextended CP.

Referring to FIGS. 8 and 9, reference numerals 0 to 3 in grids denotecell-specific RSs, CRSs transmitted through antenna port 0 to antennaport 3, for channel measurement and data modulation. The CRSs may betransmitted to UEs across a control information region as well as a datainformation region.

Reference character D in grids denotes UE-specific RSs, DerriodulationRSs (DMRSs). The DMRSs are transmitted in a data region, that is, on aPDSCH, supporting single-antenna port transmission. The existence orabsence of a UE-specific RS, DMRS is indicated to a UE by higher-layersignaling. In FIGS. 8 and 9, the DMRSs are transmitted through antennaport 5. 3GPP TS 36.211 defines DMRSs for a total of eight antenna ports,antenna port 7 to antenna port 14.

FIG. 10 illustrates an exemplary DL DMRS allocation defined in a current3GPP standard specification.

Referring to FIG. 10, DMRSs for antenna ports 7, 8, 11, and 13 aremapped using sequences for the respective antenna ports in a first DMRSgroup (DMRS Group 1), whereas DMRSs for antenna ports 9, 10, 12, and 14are mapped using sequences for the respective antenna ports in a secondDMRS group (DMRS Group 2).

As compared to CRS, CSI-RS was proposed for channel measurement of aPDSCH and up to 32 different resource configurations are available forCSI-RS to reduce Inter-Cell Interference (ICI) in a multi-cellularenvironment.

A different CSI-RS (resource) configuration is used according to thenumber of antenna ports and adjacent cells transmit CSI-RSs according todifferent (resource) configurations, if possible. Unlike CRS, CSI-RSsupports up to eight antenna ports and a total of eight antenna portsfrom antenna port 15 to antenna port 22 are allocated to CSI-RS in the3GPP standard. [Table 1] and [Table 2] list CSI-RS configurationsdefined in the 3GPP standard. Specifically, [Table 1] lists CSI-RSconfigurations in the case of a normal CP and [Table 2] lists CSI-RSconfigurations in the case of an extended CP.

TABLE 1 CSI Number of CSI reference signals configured reference signal1 or 2 4 8 Configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 Frame 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 structure 1 (11, 2) 1 (11, 2)  1 (11, 2)  1 type 1 and 2 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 3 (7,2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 06 (10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5)1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3, 2) 115 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Frame 20(11, 1)  1 (11, 1)  1 (11, 1)  1 structure 21 (9, 1) 1 (9, 1) 1 (9, 1) 1type 2 only 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 23 (10, 1)  1 (10, 1)  1 24(8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28 (3, 1)1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

TABLE 2 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 Frame structure type 1 and 2 0 (11, 4)  0 (11, 4)  0(11, 4)  0 1 (9, 4) 0 (9, 4) 0 (9, 4) 0 2 (10, 4)  1 (10, 4)  1 (10, 4) 1 3 (9, 4) 1 (9, 4) 1 (9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 0 6(4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4) 011 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 Framestructure type 2 only 16 (11, 1)  1 (11, 1)  1 (11, 1)  1 17 (10, 1)  1(10, 1)  1 (10, 1)  1 18 (9, 1) 1 (9, 1) 1 (9, 1) 1 19 (5, 1) 1 (5, 1) 120 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, 1) 1 23 (7, 1) 1 24(6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

In [Table 1] and [Table 2], (k′,l′) represents an RE index where k′ is asubcarrier index and l′ is an OFDM symbol index. FIG. 11 illustratesCSI-RS configuration #0 of DL CSI-RS configurations defined in thecurrent 3GPP standard.

In addition, CSI-RS subframe configurations may be defined, each by aperiodicity in subframes, T_(CSI-RS) and a subframe offset Δ_(CSI-RS).[Table 3] lists CSI-RS subframe configurations defined in the 3GPPstandard.

TABLE 3 CSI-RS periodicity CSI-RS subframe offset CSI-RS-SubframeConfigT_(CSI-RS) Δ_(CSI-RS) I_(CSI-RS) (subframes) (subframes) 0-4 5I_(CSI-RS)  5-14 10 I_(CSI-RS) − 5  15-34 20 I_(CSI-RS) − 15 35-74 40I_(CSI-RS) − 35  75-154 80 I_(CSI-RS) − 75

Information about a Zero Power (ZP) CSI-RS is transmitted in aCSI-RS-Config-r10 message configured as illustrated in [Table 4] by RRClayer signaling. Particularly, a ZP CSI-RS resource configurationincludes zeroTxPowerSubframeConfig-r10 and a 16-bit bitmap,zeroTxPowerResourceConfigList-r10. zeroTxPowerSubframeConfig-r10indicates the CS-RS transmission periodicity and subframe offset of a ZPCSI-RS by I_(CSI-RS) illustrated in [Table 3].zeroTxPowerResourceConfigList-r10 indicates a ZP CSI-RS configuration.The elements of this bitmap indicate the respective configurationswritten in the columns for four CSI-RS antenna ports in [Table 1] or[Table 2]. That is, the current 3GPP standard defines a ZP CSI-RS onlyfor four CSI-RS antenna ports.

TABLE 4 -- ASN1START CSI-RS-Config-r10 ::= SEQUENCE {   csi-RS-r10  CHOICE {     ...   }   zeroTxPowerCSI-RS-r10   CHOICE {     release    NULL,     setup     SEQUENCE {      zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE (16)),      zeroTxPowerSubframeConfig-r10 INTEGER (0..154)     }   } }   } --ASN1STOP

The current 3GPP standard defines modulation orders and cording ratesfor respective CQI indexes as illustrated in [Table 5].

TABLE 5 CQI index modulation code rate × 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

A CQI is calculated based on interference measurement as follows.

A UE needs to measure a Signal to Interference and Noise Ratio (SINR)for CQI calculation. In this case, the UE may measure the receptionpower (S-measure) of a desired signal in an RS such as a Non-Zero Power(NZP) CSI-RS. For interference power measurement (I-measure orInterference Measurement (IM)), the UE measures the power of aninterference signal resulting from eliminating the desired signal from areceived signal.

CSI measurement subframe sets C_(CSI,0) and C_(CSI,1) may be configuredby higher-layer signaling and the subframes of each subframe set aredifferent from the subframes of the other subframe set. In this case,the UE may perform S-measure in an RS such as a CSI-RS without anyspecific subframe constraint. However, the UE should calculate CQIsseparately for the CSI measurement subframe C_(CSI,0) and C_(CSI,1)through separate I-measures in the CSI measurement subframe setsC_(CSI,0) and C_(CSI,1).

Hereinbelow, transmission modes for a DL data channel will be described.

A current 3GPP LTE standard specification, 3GPP TS 36.213 defines DLdata channel transmission modes as illustrated in [Table 6] and [Table7]. A DL data channel transmission mode is indicated to a UE byhigher-layer signaling, that is, RRC signaling.

TABLE 6 Transmission Transmission scheme of PDSCH mode DCI formatcorresponding to PDCCH Mode 1 DCI format 1A Single-antenna port, port 0DCI format 1 Single-antenna port, port 0 Mode 2 DCI format 1A Transmitdiversity DCI format 1 Transmit diversity Mode 3 DCI format 1A Transmitdiversity DGI format 2A Large delay CDD or Transmit diversity Mode 4 DCIformat 1A Transmit diversity DCI format 2 Closed-loop spatialmultiplexing or Transmit diversity Mode 5 DCI format 1A Transmitdiversity DCI format 1D Multi-user MIMO Mode 6 DCI format 1A Transmitdiversity DCI format 1B Closed-loop spatial multiplexing using a singletransmission layer Mode 7 DCI format 1A If the number of PBCH antennaports is one, Single-antenna port, port 0 is used, otherwise Transmitdiversity DCI format 1 Single-antenna port, port 5 Mode 8 DCI format 1AIf the number of PBCH antenna ports is one, Single-antenna port, port 0is used, otherwise Transmit diversity DCI format 2B Dual layertransmission, port 7 and 8 or single-antenna port, port 7 or 8 Mode 9DCI format 1A Non-MBSFN subframe: If the number of PBCH antenna ports isone, Single- antenna port, port 0 is used, otherwise Transmit diversityMBSFN subframe: Single-antenna port, port 7 DCI format 2C Up to 8 layertransmission, ports 7-14 or single-antenna port, port 7 or 8 Mode 10 DCIformat 1A Non-MBSFN subframe: If the number of PBCH antenna ports isone, Single- antenna port, port 0 is used, otherwise Transmit diversityMBSFN subframe: Single-antenna port, port 7 DCI format 2D Up to 8 layertransmission, ports 7-14 or single-antenna port, port 7 or 8

TABLE 7 Transmission Transmission scheme of PDSCH mode DCI formatcorresponding to PDCCH Mode 1 DCI format 1A Single-antenna port, port 0DCI format 1 Single-antenna port, port 0 Mode 2 DCI format 1A Transmitdiversity DCI format 1 Transmit diversity Mode 3 DCI format 1A Transmitdiversity DCI format 2A Transmit diversity Mode 4 DCI format 1A Transmitdiversity DCI format 2 Transmit diversity Mode 5 DCI format 1A Transmitdiversity Mode 6 DCI format 1A Transmit diversity Mode 7 DCI format 1ASingle-antenna port, port 5 DCI format 1 Single-antenna port, port 5Mode 8 DCI format 1A Single-antenna port, port 7 DCI format 2BSingle-antenna port, port 7 or 8 Mode 9 DCI format 1A Single-antennaport, port 7 DCI format 2C Single-antenna port, port 7 or 8, Mode 10 DCIformat 1A Single-antenna port, port 7 DCI format 2D Single-antenna port,port 7 or 8,

Referring to [Table 6] and [Table 7], the 3GPP LTE standardspecification defines DCI formats according to the types of RNTIs bywhich a PDCCH is masked. Particularly for C-RNTI and SPS C-RNTI, the3GPP LTE standard specification defines transmission modes and DCIformats corresponding to the transmission modes, that is, transmissionmode-based DCI formats as illustrated in [Table 6] and [Table 7]. DCIformat 1A is additionally defined for application irrespective oftransmission modes, that is, for a fall-back mode. [Table 6] illustratestransmission modes for a case where a PDCCH is masked by a C-RNTI and[Table 7] illustrates transmission modes for a case where a PDCCH ismasked by an SPS C-RNTI.

Referring to [Table 6], if a UE detects DCI format 1B by blind-decodinga PDCCH masked by a C-RNTI, the UE decodes a PDSCH, assuming that thePDSCH has been transmitted in a single layer by closed-loop spatialmultiplexing.

In [Table 6] and [Table 7], Mode 10 is a DL data channel transmissionmode for CoMP. For example, in [Table 6], if the UE detects DCI format2D by blind-decoding a PDCCH masked by a C-RNTI, the UE decodes a PDSCH,assuming that the PDSCH has been transmitted through antenna port 7 toantenna port 14, that is, based on DM-RSs by a multi-layer transmissionscheme, or assuming that the PDSCH has been transmitted through a singleantenna port, DM-RS antenna port 7 or 8.

Now a description will be given of Quasi Co-Location (QCL).

If one antenna port is quasi co-located with another antenna port, thismeans that a UE may assume that the large-scale properties of a signalreceived from one of the antenna ports (or a radio channel correspondingto the antenna port) are wholly or partially identical to those of asignal received from the other antenna port (or a radio channelcorresponding to the antenna port). The large-scale properties mayinclude Doppler spread, Doppler shift, timing offset-related averagedelay, delay spread, average gain, etc.

According to the definition of QCL, the UE may not assume that antennaports that are not quasi co-located with each other have the samelarge-scaled properties. Therefore, the UE should perform a trackingprocedure independently for the respective antenna ports in order to thefrequency offsets and timing offsets of the antenna ports.

On the other hand, the UE may performing the following operationsregarding quasi co-located antenna ports.

1) The UE may apply the estimates of a radio channel corresponding to aspecific antenna port in power-delay profile, delay spread, Dopplerspectrum, and Doppler spread to Wiener filter parameters used in channelestimation of a radio channel corresponding another antenna port quasico-located with the specific antenna port.

2) The UE may acquire time synchronization and frequency synchronizationof the specific antenna port to the quasi co-located antenna port.

3) Finally, the UE may calculate the average of Reference SignalReceived Power (RSRP) measurements of the quasi co-located antenna portsto be an average gain.

For example, it is assumed that upon receipt of DM-RS-based DL datachannel scheduling information, for example, DCI format 2C on a PDCCH(or an Enhanced PDCCH (E-PDCCH)), the UE performs channel estimation ona PDSCH using a DM-RS sequence indicated by the scheduling informationand then demodulates data.

In this case, if an antenna port configured for a DM-RS used in DL datachannel estimation is quasi co-located with an antenna port for anantenna port configured for a CRS of a serving cell, the UE may useestimated large-scale properties of a radio channel corresponding to theCRS antenna port in channel estimation of a radio channel correspondingto the DM-RS antenna port, thereby increasing the reception performanceof the DM-RS-based DL data channel.

Likewise, if the DM-RS antenna port for DL data channel estimation isquasi co-located with the CSI-RS antenna port of the serving cell, theUE may use estimated large-scale properties of the radio channelcorresponding to the CSI-RS antenna port in channel estimation of theradio channel corresponding to the DM-RS antenna port, therebyincreasing the reception performance of the DM-RS-based DL data channel.

In LTE, it is regulated that when a DL signal is transmitted in Mode 10being a CoMP transmission mode, an eNB configures one of QCL type A andQCL type B for a UE.

QCL type A is based on the premise that a CRS antenna port, a DM-RSantenna port, and a CSI-RS antenna port are quasi co-located withrespect to large-scale properties except average gain. This means thatthe same node transmits a physical channel and signals. On the otherhand, QCL type B is defined such that up to four QCL modes areconfigured for each UE by a higher-layer message to enable CoMPtransmission such as DPS or JT and a QCL mode to be used for DL signaltransmission is indicated to the UE dynamically by DCI.

DPS transmission in the case of QCL type B will be described in greaterdetail.

If node #1 having N1 antenna ports transmits CSI-RS resource #1 and node#2 having N2 antenna ports transmits CSI-RS resource #2, CSI-RS resource#1 is included in QCL mode parameter set #1 and CSI-RS resource #2 isincluded in QCL mode parameter set #2. Further, an eNB configures QCLmode parameter set #1 and CSI-RS resource #2 for a UE located within thecommon overage of node #1 and node #2 by a higher-layer signal.

Then, the eNB may perform DPS by configuring QCL mode parameter set #1for the UE when transmitting data (i.e. a PDSCH) to the UE through node#1 and QCL mode parameter set #2 for the UE when transmitting data tothe UE through node #2 by DCI. If QCL mode parameter set #1 isconfigured for the UE, the UE may assume that CSI-RS resource #1 isquasi co-located with a DM-RS and if QCL mode parameter set #2 isconfigured for the UE, the UE may assume that CSI-RS resource #2 isquasi co-located with the DM-RS.

An Active Antenna System (AAS) and Three-Dimensional (3D) beamformingwill be described below.

In a legacy cellular system, an eNB reduces ICI and increases thethroughput of UEs within a cell, for example, SINRs at the UEs bymechanical tilting or electrical tilting, which will be described belowin greater detail.

FIG. 12 illustrates antenna tilting schemes. Specifically, FIG. 12( a)illustrates an antenna configuration to which antenna tilting is notapplied, FIG. 12( b) illustrates an antenna configuration to whichmechanical tilting is applied, and FIG. 12( c) illustrates an antennaconfiguration to which both mechanical tilting and electrical titlingare applied.

A comparison between FIGS. 12( a) and 12(b) reveals that mechanicaltilting suffers from a fixed beam direction at initial antennainstallation as illustrated in FIG. 12( b). On the other hand,electrical tilting allows only a very restrictive vertical beamfouningdue to cell-fixed tilting, despite the advantage of a tilting anglechangeable through an internal phase shifter as illustrated in FIG. 12(c).

FIG. 13 is a view comparing an antenna system of the related art with anAAS. Specifically, FIG. 13( a) illustrates the antenna system of therelated art and FIG. 13( b) illustrates the AAS.

Referring to FIG. 13, as compared to the antenna system of the relatedart, each of a plurality of antenna modules includes a Radio Frequency(RF) module such as a Power Amplifier (PA), that is, an active device inthe AAS. Thus, the AAS may control the power and phase on an antennamodule basis.

In general, a linear array antenna (i.e. a one-dimensional arrayantenna) such as a ULA is considered as a MIMO antenna structure. A beamthat may be formed by the one-dimensional array antenna exists on aTwo-Dimensional (2D) plane. The same thing applies to a Passive AntennaSystem (PAS)-based MIMO structure. Although a PAS-based eNB has verticalantennas and horizontal antennas, the vertical antennas may not form abeam in a vertical direction and may allow only the afore-describedmechanical tilting because the vertical antennas are in one RF module.

However, as the antenna structure of an eNB has evolved to an AAS, RFmodules are configured independently even for vertical antennas.Consequently, vertical beamforming as well as horizontal beamforming ispossible. This is called elevation beamforming.

The elevation beamforming may also be referred to as 3D beamforming inthat available beams may be formed in a 3D space along the vertical andhorizontal directions. That is, the evolution of a one-dimensional arrayantenna structure to a 2D array antenna structure enables 3Dbeamforming. 3D beamforming is not possible only when an antenna arrayis planar. Rather, 3D beamforming is possible even in a ring-shaped 3Darray structure. A feature of 3D beamforming lies in that a MIMO processtakes place in a 3D space in view of various antenna layouts other thanexisting one-dimensional antenna structures.

FIG. 14 illustrates an exemplary UE-specific beamforming in an AAS.Referring to FIG. 14, even though a UE moves forward or backward from aneNB as well as to the left and right of the eNB, a beam may be formedtoward the UE by 3D beamforming. Therefore, higher freedom is given toUE-specific beamforming.

Further, an outdoor to outdoor environment where an outdoor eNBtransmits a signal to an outdoor UE, an Outdoor to Indoor (02I)environment where an outdoor eNB transmits a signal to an indoor UE, andan indoor to indoor environment (an indoor hotspot) where an indoor eNBtransmits a signal to an indoor UE may be considered as transmissionenvironments using an AAS-based 2D array antenna structure.

FIG. 15 illustrates an AAS-based 2D beam transmission scenario.

Referring to FIG. 15, an eNB needs to consider vertical beam steeringbased on various UE heights in relation to building heights as well asUE-specific horizontal beam steering in a real cell environment wherethere are multiple buildings in a cell. Considering this cellenvironment, very different channel characteristics from those of anexisting wireless channel environment, for example, shadowing/path losschanges according to different heights, varying fading characteristics,etc. need to be reflected.

In other words, 3D beamforming is an evolution of horizontal-onlybeamforming based on an existing linear one-dimensional array antennastructure. 3D beamforming refers to a MIMO processing scheme performedby extending to or combining with elevation beamforming or verticalbeamforming using a multi-dimensional array antenna structure such as aplanar array.

Now a description will be given of a MIMO system using linear precoding.A DL MIMO system may be modeled as [Equation 11] in frequency units(e.g. a subcarriers) that are assumed to experience flat fading in thefrequency domain in a narrow band system or a wideband system.

y=Hx+z  [Equation 11]

If the number of Rx antenna ports at a UE is N_(r) and the number of Txantenna ports at an eNB is N_(t), y is an N_(r)×1 signal vector receivedat the N_(r) Rx antennas of the UE, H is a MIMO channel matrix of sizeN_(r)×N_(t), x is N_(t)×1 transmission signals, and z is an N_(r)×1received noise and interference vector in [Equation 11].

The above system model is applicable to a multi-user MIMO scenario aswell as a single-user MIMO scenario. While N_(r) is the number of Rxantennas at the single UE in the single-user MIMO scenario, N_(r) may beinterpreted as the total number of Rx antennas at multiple UEs in themulti-user MIMO scenario.

The above system model is applicable to a UL transmission scenario aswell as a DL transmission scenario. Then, N_(t) may represent the numberof Tx antennas at the UE and N_(r) may represent the number of Rxantennas at the eNB.

In the case of a linear MIMO precoder, the MIMO precoder may begenerally represented as a matrix U of size N_(t)×N_(s) where N_(s) is atransmission rank or the number of transmission layers. Accordingly, thetransmission signal vector x may be modeled as [Equation 12].

$\begin{matrix}{x = {\sqrt{\frac{P_{T}}{N_{s}}}{Us}}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

where P_(T) is transmission signal energy and s is an N_(s)×1transmission signal vector representing signals transmitted in N_(s)transmission layers. That is, E{s^(H)U^(H)Us}=N_(s). Let N_(t)×1precoding vectors corresponding to the N_(s) transmission layers bedenoted by u₁, . . . , u_(Ns). Then, U=[u₁ . . . u_(Ns)]. In this case,[Equation 12] may be expressed as [Equation 13].

$\begin{matrix}{x = {\sqrt{\frac{P_{T}}{N_{s}}}{\sum\limits_{i = 1}^{N_{s}}{u_{i}s_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

where s_(i) is an i^(th) element of the vector s. Generally, it may beassumed that signals transmitted in different layers are uncorrelated(E{s_(j)*s_(i)}=0∀i≠j) and the average magnitude of each signal is thesame. If it is assumed that the average energy of each signal is 1(E{|s_(i)|²}=1∀i), for the convenience of description, the sum of theenergy of the layer precoding vectors is N_(s) given as [Equation 14].

$\begin{matrix}{{\sum\limits_{i = 1}^{N_{s}}{E\left\{ {u_{i}^{H}u_{i}} \right\}}} = N_{s}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

If a signal is to be transmitted with the same power in each layer, itis noted from [Equation 14] that E{u_(i) ^(H)u_(i)}=1.

As a future multi-antenna system such as massive MIMO or large-scaleMIMO evolves, the number of antennas will increase gradually. In fact,use of up to 64 Tx antennas is considered for an eNB in the LTEstandard, taking into account a 3D MIMO environment. The massive antennaarray may have one or more of the following characteristics. 1) Thearray of antennas is allocated on a 2 dimensional plane or on a 3dimensional space. 2) The number of logical or physical antennas isgreater than 8. (An antenna port may refers to a logical antenna). 3)More than one antenna includes active components, i.e. activeantenna(s). But, the definition of the massive antenna array does notlimited the above-mentioned 1)˜3).

However, as the number of antennas increases, pilot overhead andfeedback overhead also increase. As a result, decoding complexity may beincreased. Since the size of the MIMO channel matrix H increases withthe number of antennas at an eNB, the eNB should transmit moremeasurement pilots to a UE so that the UE may estimate the MIMOchannels. If the UE feeds back explicit or implicit information aboutthe measured MIMO channels to the eNB, the amount of feedbackinformation will increase as the channel matrix gets larger.Particularly when a codebook-based PMI feedback is transmitted as in theLTE system, the increase of antennas in number leads to an exponentialincrease in the size of a PMI codebook. Consequently, the computationcomplexity of the eNB and the UE is increased.

In this environment, system complexity and overhead may be mitigated bypartitioning total Tx antennas and thus transmitting a pilot signal or afeedback on a sub-array basis. Especially from the perspective of theLTE standard, a large-scale MIMO system may be supported by reusing mostof the conventional pilot signal, MIMO precoding scheme, and/or feedbackscheme that support up to 8 Tx antennas.

From this viewpoint, if each layer precoding vector of the above MIMOsystem model is partitioned into M sub-precoding vectors and thesub-precoding vectors of a precoding vector for an i^(th) layer aredenoted by u_(i,1), . . . , u_(i,M), the precoding vector for the i^(th)layer may be represented as u_(i)=[u_(i,1) ^(T) u_(i,2) ^(T) . . .u_(i,M) ^(T)]^(T).

Each sub-precoding vector experiences, as effective channels, asub-channel matrix including Tx antennas in a partition corresponding tothe sub-precoding vector, obtained by dividing the N_(r)×N_(t) MIMOchannel matrix H by rows. The MIMO channel matrix H is expressed usingthe sub-channel matrices, as follows.

H=[H ₁ . . . H _(M)]  [Equation 15]

If the UE determines each preferred sub-precoding vector based on a PMIcodebook, an operation for normalizing each sub-precoding vector isneeded. Normalization refers to an overall operation for processing thevalue, size, and/or phase of a precoding vector or a specific element ofthe precoding vector in such a manner that sub-precoding vectors of thesame size may be selected from a PMI codebook for the same number of Txantennas.

For example, if the first element of the PMI codebook is 0 or 1, thephase and size of each sub-precoding vector may be normalized withrespect to 0 or 1. Hereinbelow, it is assumed that a sub-precodingvector u_(i,m) for an m^(th) partition is normalized with respect to avalue of α_(i,m) and the normalized sub-precoding vector, or theNormalized Partitioned Precoder (NPP) is v_(i,m)=u_(i,m)/α_(i,m).Therefore, partitioned precoding is modeled as [Equation 16], inconsideration of codebook-based precoding.

u _(i)=[α_(i,1) v _(i,1) ^(T)α_(i,2) v _(i,2) ^(T) . . . α_(i,M) v_(i,M) ^(T)]^(T)  [Equation 16]

As noted from [Equation 16], the values of α_(i,m) may be interpreted asvalues that link the NPPs to each other from the perspective of thewhole precoder. Hereinafter, these values will be referred to as linkingcoefficients. Thus, a precoding method for the total Tx antennas(antenna ports) may be defined by defining NPPs for the partitions ofantenna ports and linking coefficients that link the NPPs to oneanother.

M linking coefficients for the i^(th) layer may be defined as a vectora_(i)=[α_(i,1) α_(i,2) . . . α_(i,M)]^(T). Herein, a_(i) will bereferred to as a ‘linking vector’.

While it may be said that the linking vector is composed of M values,the other (M−1) values b_(i) normalized with respect to the firstelement of the linking vector may be regarded as the linking vector.That is, the relative differences of the other (M−1) NPPs with respectto the first NPP may be defined as a linking vector as expressed in[Equation 17]. This is because it is assumed in many cases that thefirst element is already normalized from the perspective of the wholeprecoding vector u_(i).

$\begin{matrix}{\frac{a_{i}}{\alpha_{i,1}} = {\left\lbrack {1\frac{\alpha_{i,2}}{\alpha_{i,1}}\frac{\alpha_{i,3}}{\alpha_{i,1}}\mspace{14mu} \ldots \mspace{14mu} \frac{\alpha_{i,M}}{\alpha_{i,1}}} \right\rbrack^{T} = \left\lbrack {1\; b_{i}^{T}} \right\rbrack^{T}}} & \left\lbrack {{Equation}\mspace{14mu} 17} \right\rbrack\end{matrix}$

If each of the transmission layers is divided into the same number ofpartitions, a linking matrix expressed as [Equation 18] may also bedefined. An NPP for each partition in the form of a matrix may bedefined as [Equation 19].

A=[a ₁ . . . a _(N) _(s) =]  [Equation 18]

V_(m)=[v_(1,m) . . . v_(N) _(s) _(,m)],m=1, . . . ,M  [Equation 19]

Let a vector obtained by repeating each element of an M×1 linking vectoras many times as the size of each partition be denoted by an extendedlinking vector â_(i). For example, if M=2 and the sizes of the first andsecond partitions are 3 and 4, respectively for an i^(th) layer,â_(i)=[α_(i,1) α_(i,1) α_(i,1) α_(i,2) α_(i,2) α_(i,2) α_(i,2)]^(T). Anextended linking matrix Â=[a₁ . . . â_(N) _(s) ] may be defined bystacking the extended linking vectors.

In this case, the whole precoding matrix may be expressed as a Hadamardproduct (or element-wise product) between the extended linking matrixand the NPP matrix V_(t) in [Equation 20].

U=Â∘V _(t)  [Equation 20]

where V_(t)=[V₁ ^(T) . . . V_(M) ^(T)]^(T) and the matrix operator ∘represents the Hadamard product.The (extended) linking vectors and the (extended) linking matrix arecollectively called a linking precoder. The term precoder is used hereinbecause the (extended) linking vectors and the (extended) linking matrixare elements determining the Tx antenna precoder. As noted from[Equation 20], one linking precoder may be configured, which should notbe construed as limiting the present invention. For example, a pluralityof sub-linking vectors may be configured by additional partitioning ofthe linking vector a_(i) and sub-linking precoders may be definedaccordingly. While the following description is given in the context ofa single linking precoder, a linking precoder partitioning scenario isnot excluded.

While the linking coefficients are represented in such a manner thatdifferent linking coefficients are applicable to different transmissionlayers in the same partition, if each layer is partitioned in the samemanner, the linking coefficients may be configured independently of thetransmission layers. That is, the same linking coefficients may beconfigured for every layer. In this case, the relationship that a□a₁= .. . =a_(N) _(s) is established between the linking vectors. Then thelinking precoder may be expressed only with M or (M−1) linkingcoefficients.

MIMO precoding schemes may be categorized largely into closed-loopprecoding and open-loop precoding. When a MIMO precoder is configured,channels between a transmitter and a receiver are considered in theclosed-loop precoding scheme. Therefore, additional overhead such astransmission of a feedback signal from a UE or transmission of a pilotsignal is required so that the transmitter may estimate MIMO channels.If the channels are accurately estimated, the closed-loop precodingscheme outperforms the open-loop precoding scheme. Thus, the closed-loopprecoding scheme is used mainly in a static environment experiencinglittle channel change between a transmitter and a receiver (e.g. anenvironment with a low Doppler spread and a low delay spread) becausethe closed-loop precoding scheme requires channel estimation accuracy.On the other hand, the open-loop precoding scheme outperforms theclosed-loop precoding scheme in an environment experiencing a greatchannel change between a transmitter and a receiver because there is nocorrelation between the channel change between the transmitter and thereceiver and a MIMO precoding scheme.

To apply closed-loop precoding to a massive MIMO environment having alarge number of antennas, information about each sub-precoder andinformation about a linking precoder are required. Withoutcodebook-based feedback, the linking precoder information may not beneeded. Depending on a partitioning method, effective channelsexperienced by each sub-precoder may have different characteristics fromeffective channels experienced by the linking precoder.

For example, one sub-precoder may experience MIMO channels having arelatively low Doppler spread, whereas another sub-precoder mayexperience MIMO channels having a relatively high Doppler spread. Inanother example, while all sub-precoders may experience effectivechannels having similar Doppler characteristics, the linking precodermay experience effective channels having different Dopplercharacteristics. Accordingly, the present invention provides a factionalbeamforming scheme that optimizes MIMO transmission adaptively accordingto the characteristics of each partitioned channel and a linking channelin the partitioned precoding environment.

Embodiment 1 Fractional Beamforming

An eNB may apply closed-loop precoding only to a part of precoders forpartitions of antenna ports and a linking precoder that links theantenna port partitions to one another and may apply one of thefollowing precoding schemes to the remaining part of the remaining partof the precoders and the linking precoder.

-   -   1. System-set precoding (hereinafter, referred to as default        precoding);    -   2. Precoding preset by an eNB or a network (hereinafter,        referred to as reference precoding); and    -   3. Precoding randomly selected by an eNB (hereinafter, referred        to as random precoding).

A set of partitions and/or linking coefficients to which closed-loopprecoding is applied is referred to as a controlled space and a set ofpartitions and/or linking coefficients to which closed-loop precoding isnot applied is referred to as an uncontrolled space.

In default precoding, the system defines a beam for transmission in theuncontrolled space. It may be regulated that default precoding followsopen-loop precoding. A different default precoding scheme may be setaccording to a system bandwidth, the number of Tx antennas at an eNB,the number of transmission layers (or a transmission rank), a Tx antennaconfiguration of the eNB (N_(t) _(—) _(v), N_(t) _(—) _(h)), or thenumber of Tx antennas directed in an uncontrolled direction. Or aspecific beam may be set irrespective of the system parameters in thedefault precoding scheme. In addition, the default precoding scheme maybe fixed across a total frequency band and a total time area or may bechanged on a predetermined time resource unit basis and/or apredetermined frequency resource unit basis.

In reference precoding, the eNB or the network configures a precodingscheme to be applied to the uncontrolled space for a UE. Accordingly,reference precoding information for the uncontrolled space istransmitted to the UE by a physical layer message or a higher layermessage. The reference precoding information is any information thatindicates a MIMO precoder to be applied to the uncontrolled spaceimplicitly or explicitly. For example, the reference precodinginformation may include a specific index (PMI) of a PMI codebookcorresponding to the number of uncontrolled space Tx antennas, thequantized value of each element of a MIMO precoding matrix for theuncontrolled space, and an index for use in transmission, selected fromamong the indexes of a plurality of MIMO precoding schemes.

Reference precoding may also be changed on a predetermined time resourceunit basis and/or a predetermined frequency resource unit basis. In thiscase, a plurality of reference precoding patterns that change intime/frequency resources are defined and then the index of a referenceprecoding pattern used by the eNB or the network may be signaled asreference precoding information. Or a seed value of a random variablegenerator that may induce reference precoding patterns that change intime/frequency resources may be used as reference precoding information.Or reference precoding information may be configured to indicate a usedprecoding scheme selected from among various precoding schemes (e.g.Space Time Block Coding (STBC), delay diversity, etc.).

In random precoding, the eNB randomly selects a precoding scheme for theuncontrolled space. Therefore, compared to default precoding orreference precoding, the UE does not have knowledge of a precoder to beapplied to the uncontrolled space. For example, the eNB may transmit abeam that changes randomly in the uncontrolled space on a predeterminedtime resource basis (e.g. on an OFDM symbol basis) and/or apredetermined frequency resource unit basis (e.g. on a subcarrierbasis).

According to the fractional beamforming method in the embodiment of thepresent invention, independent partitioning and fractional beamformingmay be applied to each transmission layer. Or the same partitioning andbeamforming scheme may be applied to all transmission layers.

The fractional beamforming method of the present invention is veryuseful, when the reliability of feedback information about a part of Txantennas or the reliability of feedback information about linkingcoefficients is low or in a channel environment that does not requiresuch a feedback. Especially when the reliability of feedback informationabout a part of Tx antennas or the reliability of feedback informationabout linking coefficients is low, the fractional beamforming method isadvantageous in that a packet reception error and unnecessary packetretransmission caused by a feedback information error can be prevented.In addition, when the feedback is unnecessary, the fractionalbeamforming method can minimize feedback overhead.

Embodiment 2 Aliened Fractional Precoding

If a part or all of antenna port partitions are of the same size andcorresponding partitioned antenna arrays have similar effective channelcharacteristics, the same precoding scheme, that is, aligned fractionalprecoding may be applied to corresponding NPPs.

FIG. 16 illustrates an example of applying aligned fractional precodingto a Uniform Linear Array (ULA) according to another embodiment of thepresent invention.

Referring to FIG. 16, in a ULA with 8 antennas, a first partition(Partition 1) includes 1^(st), 3^(rd), 5^(th) and 7^(th) antennas and asecond partition (Partition 2) includes 2^(nd), 4^(th), 6^(th), and8^(th) antennas. If the gap between antennas is narrow and there are notmany scatterers around the ULA, Partition 1 and Partition 2 are highlylikely to experience similar MIMO channels except for a phase differencebetween the two partitions, corresponding to a linking precodercomponent. In this case, the same precoding scheme is configured for thetwo partitions.

FIG. 17 illustrates an example of applying columnwise aligned fractionalprecoding to a square array according to another embodiment of thepresent invention.

Referring to FIG. 17, each column is set as one partition in a squarearray having N_(t)(=N_(t) _(—) _(v)×N_(t) _(—) _(h)) antennas arrangedin N_(t) _(—) _(v) rows and N_(t) _(—) _(h) columns. If the gap betweencolumns is narrow and N_(t) _(—) _(h) is not large, the same precodingscheme may be configured for all partitions. However, a linking vectoris set independently of the sub-precoder.

FIG. 18 illustrates an example of applying rowwise aligned fractionalprecoding to a square array according to another embodiment of thepresent invention.

Referring to FIG. 18, each row is set as one partition in a square arrayhaving N_(t)(=N_(t) _(—) _(v)×N_(t) _(—) _(h)) antennas arranged inN_(t) _(—) _(v) rows and N_(t) _(—) _(h) columns. If the gap betweenrows is narrow and N_(t) _(—) _(v) is not large, the same precodingscheme may be configured for all partitions. However, a linking vectoris set independently of the sub-precoder.

FIG. 19 illustrates an example of applying row groupwise alignedfractional precoding to a square array according to another embodimentof the present invention.

Referring to FIG. 19, each row group including N rows is set as onepartition in a square array having N_(t)(=N_(t) _(—) _(v)×N_(t) _(—)_(h)) antennas arranged in N_(t) _(—) _(v) rows and N_(t) _(—) _(h)columns. If the gap between row groups is narrow and N_(t) _(—) _(v) isnot large, the same precoding scheme may be set for all partitions.However, a linking vector is set independently of the sub-precoder.

As illustrated in FIGS. 16 to 19, if all partitions are of the same sizeand the same precoder is applied to the partitions (i.e. v_(i)□v_(i,1)=. . . =v_(i,M)), a precoder for an i^(th) layer may be represented as aKronecker product between a linking precoder and a sub-precoder, givenas [Equation 21].

u _(i)=[α_(i,1) v _(i,1) ^(T)α_(i,2) v _(i,2) ^(T) . . . α_(i,M) v_(i,M) ^(T)]^(T)=[α_(i,1) v _(i) ^(T)α_(i,2) v _(i) ^(T) . . . α_(i,M) v_(i) ^(T)]^(T) =a _(i)

v _(i)  [Equation 21]

If all transmission layers are partitioned in the same manner, a MIMOprecoder for the total layers may be represented as a Khatri-Rao product(a columnwise Kronecker product) between an M×N_(s) linking matrix A andan

$\frac{N_{t}}{M} \times N_{s}$

sub-precoding matrix V=[v₁ . . . v_(N) _(s) ], given as [Equation 22].

U=[a ₁

v ₁ . . . a _(Ns)

v _(Ns) ]=A*V  [Equation 22]

If each column is set as one partition in a Two-Dimensional (2D) antennaport array environment as illustrated in FIG. 17, vertical beamforming(or elevation beamforming) is performed using the sub-precoder v_(i) orV and horizontal beamforming (or azimuth beamforming) is performed usingthe linking precoder a_(i) or A. If each row is set as one partition ina 2D antenna port array environment as illustrated in FIG. 18,horizontal beamforming (or azimuth beamforming) is performed using thesub-precoder v₁ or V and vertical beamforming (or elevation beamforming)v is performed using the linking precoder a_(i) or A.

In the case of perfectly aligned fractional precoding in a row or columndirection in a 2D antenna (port) array environment as illustrated inFIG. 17 or FIG. 18, a precoder that performs 3D beamforming may beexpressed as one sub-precoder and one linking precoder. Verticalbeamforming is performed using one of the sub-precoder and the linkingprecoder and horizontal beamforming is performed using the otherprecoder.

If the fractional beamforming proposed for the environment of perfectlyaligned fractional precoding is used, the eNB applies closed-loopprecoding to one of a sub-precoder and a linking precoder and one ofdefault precoding, reference precoding, and random precoding to theother precoder in an environment where the same precoding is used forall partitions.

The second embodiment of the present invention is useful to 3Dbeamforming in a 2D antenna array environment as illustrated in FIGS. 17and 18. 3D beamforming, particularly UE-specific 3D beamformingadvantageously optimizes transmission performance according to thehorizontal and vertical positions of a UE and a scattering environmentof a 3D space. However, UE-specific 3D beamforming is a closed-loopprecoding scheme and thus requires accurate CSI between an eNB and a UE.

Therefore, as the number of eNB antennas and the dimension ofbeamforming increase, the difference between a minimum performance valueand a maximum performance value gets wider depending on MIMOtransmission schemes. Consequently, performance gets more sensitive to aCSI estimation error factor of an eNB, such as a channel estimationerror, a feedback error, and channel aging. If the CSI estimation errorof the eNB is not significant, normal transmission may be performed dueto channel coding or the like. On the other hand, in the case of aserious CSI estimation error in the eNB, a packet reception error occursand packet retransmission is required, thus degrading performanceconsiderably.

For example, 3D beamforming for a UE that is moving fast in a horizontaldirection with respect to an eNB increases a packet retransmissionprobability. While open-loop precoding is conventionally used for theUE, vertical beamforming is favorable for the UE because the UEexperiences a static channel in a vertical direction. On the other hand,horizontal beamforming is favorable for a UE fast moving in the verticaldirection or an environment where scattering is severe in the verticaldirection. For a UE located in a narrow, tall building, the eNB mayperform 3D beamforming with horizontal beamforming fixed to a specificdirection. That is, the UE is instructed to configure feedbackinformation only for vertical beamforming, thus reducing feedbackoverhead.

Therefore, if the fractional beamforming according to the secondembodiment of the present invention is applied to a 3D beamformingenvironment, 2D beamforming (vertical beamforming or horizontalbeamforming) may be performed according to a user environment. In thisrespect, the fractional beamforming scheme may be called partialdimensional beamforming. For example, an eNB having 2D Tx antenna portsmay apply closed-loop precoding to one of a vertical precoder and ahorizontal precoder and one of default precoding, reference precoding,and random precoding to the other precoder.

Embodiment 3

In the fractional precoding schemes according to the forgoingembodiments of the present invention, each sub-precoder and a linkingprecoder have been defined from the viewpoint of data transmission froman eNB. In regards to a sub-precoder and a linking precoder to whichclosed precoding is applied, a UE may transmit a Preferred PrecodingIndex (PPI) to an eNB. After matrix precoders are indexed, a preferredmatrix precoder index may be fed back as a PPI in a PMI feedback scheme.

If some feedback information is separated on the basis of a unitincluding a partition and/or a value linking partitions, pilot signalstransmitted from an eNB to a UE may be associated with a set of specificantenna ports. A set of such pilot signals is called a pilot pattern. Amajor pilot pattern involves Non-Zero-Power (NZP) CSI-RS resources (orprocesses) which are measurement pilots used in the LTE system. Forexample, the following mapping relationship may be established betweenpartitions, CSI-RSs, and PMI feedbacks.

A. Aligned Unit of Partition & Pilot Pattern & PMI Feedback

1. (Partition): in a system with 16 antenna ports, an eNB divides the 16antenna ports into two partitions each having 8 antenna ports andperforms fractional precoding on the two partitions.

2. (Pilot pattern): the eNB allocates 8Tx NZP CSI-RS resources to eachpartition for a UE, that is, configures two co-located NZP CSI-RSresources for the UE in order to support the fractional precoding.

3. (PMI feedback): the UE feeds back PMI1 and PMI2 for the two antennaport partitions, and linking coefficients (e.g. PMI3 for a linkingprecoder) that link PMI1 to PMI2.

That is, if an NZP CSI-RS resource is separately allocated to eachantenna port partition, the eNB may configure a plurality of NZP CSI-RSresources to the UE, for a plurality of co-located (or synchronized)antenna port partitions belonging to the eNB (or transmission point). Todistinguish a non-co-located antenna port pattern used for CoMPtransmission from the co-located antenna port patterns, the eNB mayadditionally indicate co-location or non-co-location between NZP CSI-RSresources. For example, a Quasi-Co-Location (QCL) condition between aplurality of NZP CSI-RS resources may be indicated to the UE.

A pilot transmission unit and an antenna port partition unit are notalways identical as in the above example. For example, when one 8TxCSI-RS resource is configured, the UE may configure feedback informationfor two 4Tx partitions. In addition, an antenna port partition unit anda feedback unit are not always identical. Particularly in the case ofaligned partitioned precoding, common PPI feedback information may betransmitted for partitions to which the same precoding is applied.Therefore, one feedback unit may be configured for a plurality ofpartitions.

B. Not Aligned Unit of Partition & Pilot Pattern & PMI Feedback

1. (Partition): it is assumed that antenna ports are partitioned asillustrated in FIG. 18.

2. (PMI feedback): feedback information includes a PPI commonly appliedto all partitions (referred to as a common PPI) and linkingcoefficients, in consideration of perfectly aligned fractionalprecoding. In this case, the partition unit and the feedback unit may bedifferent.

3. (Pilot pattern): a pilot pattern may be allocated in various manners.FIGS. 20, 21, and 22 illustrate exemplary pilot pattern allocationmethods according to a third embodiment of the present invention.Specifically, a pilot resource may be configured separately for eachpartition as illustrated in FIG. 20. As illustrated in FIG. 21, onepilot pattern may be transmitted in a first partition so that the UE maycalculate a common PPI, and one pilot pattern may be transmitted throughantenna ports to which a linking precoder is applied, so that the UE maycalculate linking coefficients. Or only one pilot pattern may beconfigured so that the UE may calculate a common PPI and linkingcoefficients at one time, as illustrated in FIG. 22.

Embodiment 4 CSI Calculation for Fractional Beamforming

A fourth embodiment of the present invention provides a method forcalculating CSI and a method for configuring CSI feedback information ata UE, for fractional beamforming. It is assumed as a CSI calculationmethod of a UE in a fractional beamforming system that the UE appliesone of default precoding, reference precoding, and random precoding to apart of antenna port partitions and linking coefficients, correspondingto an uncontrolled space, when the UE measures or calculates partialCSI.

The partial CSI includes a CQI and an RI as well as a PMI. In the caseof random precoding, the UE has no knowledge of a precoding scheme thatthe eNB applies to the uncontrolled space and thus the UE calculatesCSI, assuming an arbitrary precoding scheme for the uncontrolled spaceas applied by the eNB.

After the UE assumes an arbitrary precoding scheme for the uncontrolledspace, the UE may calculate CSI in the following manners.

(1) The UE sets N precoder candidates (N is a finite number) for theuncontrolled space and calculates CQIs that may be achieved using therespective candidates, CQI₁, . . . CQI_(N). Then the UE reports theaverage of the CQIs calculated for all precoder candidates for theuncontrolled space (i.e. CQI=(CQI₁+ . . . +CQI_(N))/N) to the eNB.

(2) The UE sets N precoder candidates (N is a finite number) for theuncontrolled space and calculates CQIs that may be achieved using therespective candidates, CQI₁, . . . , CQI_(N). Then the UE reports theCQI of a worst case among all precoder candidates for the uncontrolledspace (i.e. CQI=minimum of {CQI₁, . . . , CQI_(N)}) to the eNB.

(3) The UE may generate and set a random precoder for the uncontrolledspace and may calculate a CQI that may be achieved using the precoder.Then the UE may feed back the CQI to the eNB.

If the above CQI calculation methods are extended/applied to a partialdimensional beamforming technique for a 3D beamforming environment, theUE may apply one of default precoding, reference precoding, and randomprecoding to one of a vertical precoder and a horizontal precoder inmeasuring or calculating partial CSI.

While a partition viewpoint and a CSI feedback viewpoint have beenassociated in the above description, a pilot-CSI feedback relationshipmay be different from a partition-CSI feedback relationship. Therefore,the UE may apply one of default precoding, reference precoding, andrandom precoding to a part of a plurality of (co-located) antenna portpatterns and values that link the (co-located) antenna port patterns,corresponding to the uncontrolled space, in measuring or calculatingpartial CSI. The antenna part patterns cover NZP CSI-RS resources andCSI-RS patterns. This will be specified as the followings.

(A) If a linking precoder (or a vertical precoder) belongs to theuncontrolled space in the example of FIG. 20, the eNB sets a pluralityof (co-located) pilot patterns and the UE calculates CSI on theassumption that a value linking a PMI(s) to be applied to MIMO channelscorresponding to each pilot pattern is a system-set value, a value setby an eNB, or a random value.

(B) If sub-precoders (or horizontal precoders) belong to theuncontrolled space in the example of FIG. 20, the eNB sets a pluralityof (co-located) pilot patterns and the UE calculates CSI on theassumption that a precoder to be applied to a part or all of the pilotpatterns is a system-set value, a value set by an eNB, or a randomvalue.

(C) If a linking precoder (or a vertical precoder) belongs to theuncontrolled space in the example of FIG. 21, the eNB configures twoco-located pilot patterns for the UE and the UE calculates CSI on theassumption that a precoder to be applied to MIMO channels correspondingto one of the pilot patterns is a system-set value, a value set by aneNB, or a random value.

(D) The eNB configures one pilot pattern for the UE in the example ofFIG. 22, and the UE calculates CSI on the assumption that a precoder tobe applied to MIMO channels corresponding to a part of antenna portsbelonging to the pilot pattern is a system-set value, a value set by aneNB, or a random value.

Embodiment 5 CSI Contents for Fractional Beamforming

Implicit feedback information for fractional beamforming may include aUE-preferred PMI or coefficients for a part of partitions and/or alinking precoder. When configuring PPI feedback information, the UE mayinclude, as CSI contents, only a PPI for a part of a plurality of(co-located) antenna port patterns and values linking the (co-located)antenna port patterns to one another, corresponding to an uncontrolledspace, taking into account the relationship between a pilot (pattern)and a PMI feedback.

Since the (co-located) antenna port patterns belong to the sametransmission point, it is efficient to feed back a common CQI and acommon RI to an eNB. Therefore, when configuring feedback information,the UE may include, as CSI contents, a PPI for a part of a plurality of(co-located) antenna port patterns and values linking the (co-located)antenna port patterns to one another, corresponding to an uncontrolledspace, and a CQI and RI for the total (co-located) antenna portpatterns. Specifically, CSI contents may be configured in the followingmanners (a), (b), and (c).

(a) The eNB configures N (co-located) pilot patterns CSI-RS #0, . . . ,N−1 for the UE and the UE transmits PMIs for M (M<N) pilot patterns fromamong the N pilot patterns and a CQI and RI for the total antennas. TheUE may additionally feed back a PMI for a linking precoder. In thiscase, the UE may calculate PMIs, CQIs, and RIs for CSI-RS patterns forwhich PMIs are not reported to the eNB, by the CSI calculation methodaccording to the fourth embodiment of the present invention.

(b) In the CSI-RS transmission method for a 3D beamforming environment,illustrated in FIG. 21, the eNB may configure two (co-located) CSI-RSpatterns and the UE may transmit, to the eNB, a PMI for one of the twoCSI-RS patterns and a CQI and RI for aggregated CSI-RS resources of thetwo CSI-RS patterns. In this case, since the first antenna ports of thetwo CSI-RS patterns correspond to the same physical antenna, the UE doesnot transmit a PPI for a linking precoder.

(c) In the single pilot pattern configuration method illustrated in FIG.22, the eNB may configure one CSI-RS pattern for the UE and the UE maytransmit, to the eNB, a PMI for a part of the antenna ports of theCSI-RS pattern and a CQI and RI for the whole antenna ports.

While it is assumed in (a), (b), and (c) that one CQI is fed back forthe whole transmission layers, the present invention is not limited tothe specific assumption. For example, if the same Modulation and CodingScheme (MCS) is set for a plurality of layers as in the LTE system, aCQI may be fed back on a codeword basis. In this case, one CQI percodeword may be transmitted.

For fractional beamforming, information about channel movement of the UEis needed as CSI or an additional feedback. Specifically, thisinformation may include statistic information about channels (e.g. aLine Of Sight (LOS) parameter, path loss, correlation, etc.) andmobility information (movement direction, speed, acceleration, Dopplerspread, etc.).

Particularly, the movement direction may be an absolute direction (e.g.a change in a relative position with respect to a predeterminedreference position) or a relative direction (e.g. a change in theposition of the UE with respect to the position of a reference eNB). Thereference eNB position may refer to the position of a serving eNB(transmission point), the position of a predetermined eNB (transmissionpoint), or specific coordinates signaled by an eNB. Further, therelative direction may be measured based on a specific signal such as aPositioning Reference Signal(s) (PRS(s)) received from an eNB(s) or aspecific message including relative distance information or responsedelay information.

In the foregoing embodiments of the present invention, one PMI is notalways represented as a single index. For example, the LTE systemregulates that a UE feeds back two PMIs for 8 Tx antenna ports of aneNB. Accordingly, if one pilot pattern includes 8 or more Tx antennaports, two or more PMIs may be used to indicate preferred indexes foreach pilot pattern.

If feedback information configured according to the present invention isapplied to a wide band system, specific frequency areas may be defined(e.g. subbands, subcarriers, resource blocks, etc.) and a set offeedback information may be transmitted for each frequency area. Orfeedback information may be transmitted only for a specific frequencyarea selected by a UE or indicated by an eNB. The frequency area mayinclude one or more contiguous or non-contiguous frequency areas.

FIG. 23 is a block diagram of a communication apparatus according to anembodiment of the present invention.

Referring to FIG. 23, a communication apparatus 2300 includes aprocessor 2310, a memory 2320, an RF module 2330, a display module 2340,and a User Interface (UI) module 2350.

The communication device 2300 is shown as having the configurationillustrated in FIG. 23, for the convenience of description. Some modulesmay be added to or omitted from the communication apparatus 2300. Inaddition, a module of the communication apparatus 2300 may be dividedinto more modules. The processor 2310 is configured to performoperations according to the embodiments of the present inventiondescribed before with reference to the drawings. Specifically, fordetailed operations of the processor 2310, the descriptions of FIGS. 1to 22 may be referred to.

The memory 2320 is connected to the processor 2310 and stores anOperating System (OS), applications, program codes, data, etc. The RFmodule 2330, which is connected to the processor 2310, upconverts abaseband signal to an RF signal or downconverts an RF signal to abaseband signal. For this purpose, the RF module 2330 performsdigital-to-analog conversion, amplification, filtering, and frequencyupconversion or performs these processes reversely. The display module2340 is connected to the processor 2310 and displays various types ofinformation. The display module 2340 may be configured as, not limitedto, a known component such as a Liquid Crystal Display (LCD), a LightEmitting Diode (LED) display, and an Organic Light Emitting Diode (OLED)display. The UI module 2350 is connected to the processor 2310 and maybe configured with a combination of known user interfaces such as akeypad, a touch screen, etc.

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

A specific operation described as performed by a BS may be performed byan upper node of the BS. Namely, it is apparent that, in a networkcomprised of a plurality of network nodes including a BS, variousoperations performed for communication with a UE may be performed by theBS, or network nodes other than the BS. The term ‘BS’ may be replacedwith the term ‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B oreNB)’, ‘Access Point (AP)’, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to exemplaryembodiments of the present invention may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the method for performing fractional beamforming by large-scaleMIMO in a wireless communication system has been described in thecontext of a 3GPP LTE system, the present invention is also applicableto many other wireless communication systems. Further, the presentinvention is related to the massive antenna array, but is applicable toany antenna array structures.

1. A method for reporting Channel State Information (CSI) by a UserEquipment (UE), for fractional beamforming using a massive antenna arrayof a Base Station (BS) in a wireless communication system, the methodcomprising: receiving information about a plurality of Reference Signal(RS) resources from the BS; generating CSI including a sub-precoder forat least one of the plurality of RS resources, and a Channel QualityIndicator (CQI) and a Rank Indicator (RI) for all of the plurality of RSresources; and reporting the CSI to the BS, wherein the massive antennaarray is divided by rows or columns into partitions and the plurality ofRS resources correspond to the partitions.
 2. The method according toclaim 1, wherein the generation of the CSI comprises generating the CSI,assuming that one linking precoder for linking the plurality of RSresources is a predetermined value or a random value.
 3. The methodaccording to claim 1, further comprising receiving information about onelinking precoder for linking the plurality of RS resources from the BS.4. The method according to claim 1, wherein if the massive antenna arrayis divided by columns into the partitions, the sub-precoder is used forvertical beamforming and a linking precoder is used for horizontalbeamforming.
 5. The method according to claim 1, wherein if the massiveantenna array is divided by rows into the partitions, the sub-precoderis used for horizontal beamforming and a linking precoder is used forvertical beamforming.
 6. The method according to claim 1, wherein eachof the partitions includes the same number of antenna ports and a gapbetween the antenna ports is equal to or smaller than a predeterminedvalue.
 7. The method according to claim 2, wherein if the linkingprecoder is assumed to be a random value, the generation of the CSIcomprises: generating CQIs, assuming that a plurality of linkingprecoder candidates are applied respectively; and calculating an averageor a minimum value of the CQIs as the CQI for all of the plurality of RSresources.
 8. A reception apparatus in a wireless communication system,the reception apparatus comprising: a wireless communication moduleconfigured to receive information about a plurality of Reference Signal(RS) resources from a transmission apparatus that performs fractionalbeamforming using a massive antenna array and to transmit Channel StateInformation (CSI) generated using the plurality of RS resources to thetransmission apparatus; and a processor configured to generate the CSIincluding a sub-precoder for at least one of the plurality of RSresources, and a Channel Quality Indicator (CQI) and a Rank Indicator(RI) for all of the plurality of RS resources, wherein the massiveantenna array of the transmission apparatus is divided by rows orcolumns into partitions and the plurality of RS resources correspond tothe partitions.
 9. The reception apparatus according to claim 8, whereinthe processor generates the CSI, assuming that one linking precoder forlinking the plurality of RS resources is a predetermined value or arandom value.
 10. The reception apparatus according to claim 8, whereinthe wireless communication module receives information about one linkingprecoder for linking the plurality of RS resources from the transmissionapparatus.
 11. The reception apparatus according to claim 8, wherein ifthe massive antenna array is divided by columns into the partitions, thesub-precoder is used for vertical beamforming and a linking precoder isused for horizontal beamforming.
 12. The reception apparatus accordingto claim 8, wherein if the massive antenna array is divided by rows intothe partitions, the sub-precoder is used for horizontal beamforming anda linking precoder is used for vertical beamforming.
 13. The receptionapparatus according to claim 8, wherein each of the partitions includesthe same number of antenna ports and a gap between the antenna ports isequal to or smaller than a predetermined value.
 14. The receptionapparatus according to claim 9, wherein if the linking precoder isassumed to be a random value, the processor generates CQIs, assumingthat a plurality of linking precoder candidates are appliedrespectively, and calculates an average or a minimum value of the CQIsas the CQI for all of the plurality of RS resources.